1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a method of manufacturing an acoustic resonator and the resonator manufactured thereby, and in particular, to a method of manufacturing a film bulk acoustic resonator (FBAR) using internal stress of a metallic film and the resonator manufactured thereby.
2. Description of the Related Art
Mobile communication technologies have dramatically developed recently. Mobile communication technologies require various parts of the radio frequency (RF) spectrum that can effectively relay information within a limited frequency band. In particular, a filter among RF parts is one of the core parts used in mobile communication technologies for selecting signals from numerous public waves or filtering the signals to be transmitted as desired by a user, thereby realizing a communication of high quality.
The most commonly used RF filters for mobile communication are a dielectric filter and a surface acoustic wave (SAW) filter. The dielectric filter has advantages of high dielectric constant, low insertion loss, stability at high temperatures and high resistance to vibration and impact. However, the dielectric filter has limitations in minimizing its size and realizing monolithic microwave ICs (MMIC) that are recent trends in technical development.
Meanwhile, the SAW filter has advantages of being smaller than the dielectric filter and realizing easy signal processing as well as being simple in its circuit. The SAW filter can also be manufactured on a large-scale basis. Further, the SAW filter has higher side rejection within the passband than the dielectric filter, thereby realizing an exchange of information of high quality. However, the SAW filtering process includes an exposing process using an ultraviolet layer. Thus, the SAW filter has a limitation to 0.51 μm in its inter-digital transducer (IDT) line width. Accordingly, it is impossible to cover the high radio frequency (higher than 5 GHz) band by using the SAW filter. Moreover, it is fundamentally impossible to construct the SAW filter with an MMIC structure, which is adopted for a semiconductor substrate, and with a single chip.
Suggested to overcome the above limitations and problems was an FBAR filter, which can completely realize the frequency control circuits as an MMIC by being integrated with other active elements on a conventional semiconductor (Si, GaAs) substrate.
The FBAR is a thin film element incurring low manufacturing cost and has a small size with a high quality coefficient. Therefore, the FBAR can be used for mobile communication apparatuses in a broad frequency band (900 MHz˜10 GHz) and military radar, etc. The FBAR can also be miniaturized to one several hundredth of the size of the dielectric filter or an LC filter while having a considerably smaller amount of insertion loss than the SAW filter. Thus, the FBAR is highly stable and applicable to MMICs requiring a high quality coefficient.
The FBAR filter is produced by directly depositing ZnO or AIN, which is a piezodielectric material, using radio frequency (RF) sputtering on Si or GaAs, which is a semiconductor substrate so as to induce resonance due to the piezoelectric characteristic. Such FBAR induces bulk acoustic waves by depositing piezoelectric film between two electrodes so as to generate resonance.
FIGS. 1 to 3 show structures of the conventional FBARs.
FIG. 1 shows a structure of an FBAR of the conventional bulk micro machining type. In the FBAR of the bulk micro machining type, a membrane layer is formed through a cavity 16, which is formed by depositing SiO2 12 on a semiconductor substrate 11 and isotropically etching an opposite surface of the semiconductor substrate 11. A lower part electrode layer 13 is formed on an upper part of the SiO2 12. A piezoelectric layer 14 is formed by depositing a piezoelectric material on the lower electrode layer 13 by means of RF magnetron sputtering. An upper part electrode layer 15 is formed on the piezoelectric layer 14. The drawing reference numeral 17 represents a resonance structure.
Such FBAR of the bulk micro machining type has advantages of reducing loss of dielectric properties from the semiconductor substrate 11 as well as of electric power because of the cavity 16. However, the FBAR of the bulk micro machining type also has drawbacks in that an element occupies a large area due to the orientation of the semiconductor substrate and a yield rate is deteriorated due to breaking caused by low structural stability in the post packaging process. Recently introduced to overcome such problems of the FBAR of the bulk micro machining type and to simplify the process of manufacturing an element are FBARs of an air gap type and of a Bragg reflector type.
FIG. 2 shows a structure of the FBAR of a Bragg reflector type. The Bragg reflector type FBAR is manufactured by laminating an acoustic reflector 28, which comprises predetermined members 22, 23, 24, on a semiconductor substrate, and laminating a resonance structure 29 on the acoustic reflector 28. In the Bragg reflector type FBAR, a material having a great difference in acoustic impedance is deposited on the semiconductor substrate 21 in separation layers so as to induce Bragg reflection and resonance of acoustic energy between the upper and lower electrode layers 25, 27. The drawing reference numeral 26 represents a piezoelectric layer.
Thus, the advantages of the Bragg reflector type FBAR are that it has a firm structure, incurs less manufacturing time and is highly resistant to external impact. However, it also poses problems in that it is difficult to control the thickness between the layers to realize total reflection and manufacturing cost increases to form a reflection layer for the acoustic reflection 28.
FIG. 3 shows a structure of the FBAR of a surface micro machining type. In the surface micro machining type FBAR, a membrane layer 32 is formed with an air gap 36, which is generated in a sacrificial layer on the semiconductor substrate 31 by using micromachining technology. The drawing reference numerals 32, 33, 34, 35 and 36 respectively represent oxidized silicon film, a lower electrode layer, a piezoelectric layer, an upper electrode layer, and a resonance structure.
The surface micro machining type FBAR serves to reduce a long processing time consumed when etching an opposite surface of the substrate to form the membrane layer as shown in FIG. 1 as well as the danger that might be caused by harmful gas. The surface micro machining type FBAR also has merits of losing less dielectric properties of the semiconductor substrate and being small in its area. However, it has drawbacks of a deteriorating low yield rate and easy breakability in the post process because of long exposure of the structure when etching the sacrificial layer as well as being a complicated manufacturing process.